Thaddeus Thawne was a smart politician from the later 30th century who reach the highest office from his time: the Earthgov presidency. Sadly,he also was member of the Thawne family, a linage of supervillians than included Cobalt Blue and the reverse Flash. Since the 20th century, the Thawnes had opposed the power of their magic Cobalt Blue gem to the speed force generated superspeed from the Allen family, with their share of victories and fails for both sides.
President Thawne would however face an unexpected turn in the generational war when his daugther Meloni Thawne fell in love with Don Allen, son of the legendary Flash, Barry Allen. this didn't stop President Thawne from arrange the deaths of Don and his sister Dawn, the Tornado twins with the help of the aliens Dominators and take measures to end the Allen bloodline. Depiste his efforts, Don and Dawn left a legacy alive: Dawn in her daughter Jenni Ognats, apparentely a normal human child and Don with Meloni in President Thawne own grandchild Bart Allen aka Impulse.
Thawne himself put all the resources of Earthgov to secure the control of Bart, but Iris Russell help him to escape to the 20th centruy. Depiste this setback, Thawne had already keep a sample of Bart DNA, for his own purposes.
The next task for Thaddeus was reconstitute the Blue Flame Gem with the help of sorcerers from Zerox and make it possible for the Thawnes to rule the galaxy by amplifing its powers, but his plans were folded by a time traveling Wally West and by his own ancestor, Professor Zoom, who had his own plans. In the aftermath of the super-speed struggle the gem was destroyed and their fragments apparentely loss, to Thawne consternation.
After this new failure and still keeping the presidential title, Thawne set in motion another plan. With the sample of Bart’s DNA, he began to grow a clone but in a different way than Impulse. It would take centuries to develop, but as was spliced more on the thawne side, he considered than the clone would be everything than Impulse wasn't: it would be perfect. Calling the clone as himself, Thaddeus, the boy who would be know as Inertia would be trained to supplant Bart Allen and complete the subversion of the Allen line by the Thawnes, in the 20th century.
The final fate of Thaddeus Thawne is unknown. However in one possible future involved Thawne destrrying the planet. When Max Mercury died at the dawn of the 21st century, Bart and his girlfriend Carol Bucklen would have traveled to their future and settled there. Thawne would have remained in office for the next 15 years, stealing Carol’s Speed Force research to create the Hyperguard and then to irradiate the entire planet, killing most of the population and leaving the survivors with the benefits of superspeed. The few survivors include an adult Bart and Carol opposed to the tyranical rule of Thawne.
This timeline however was averted when impulse developted a new power of energy time-traveling self-replications than prevent Max’s death thus erasing all the events. To erradicate any possibility than Thawne could access to Carol investgation, the girl choose to travele with Impulse mother to the 63rd century. In the 31st century, 15 years later from that events and when the Legion of Super-Heroes was formed, Thawne was no longer in office and his whereabout were unknown.